This invention relates to the field of microwave antennas. More particularly, this invention relates to a parallel plate waveguide antenna for use as a transmitter antenna or receiver antenna primarily for broadcasted microwave signals for TV systems. While this invention may have general utility in microwave transmission or reception, the invention will be described in the preferred environment of a direct satellite broadcasting (DSB) system. However, it will be understood that the invention may have general utility as either a receiver antenna or transmitter antenna in microwave communication systems.
With the growing potential for satellite transmission of microwave signals for TV broadcasting and receiving systems, there is an increasing need for a reliable, durable and reasonably inexpensive antenna for household and other commercial use for the reception of satellite transmitted microwave signals. Parabolic antennas are traditionally used in transmission systems of this type, but they present many problems for an effective and commercially viable TV microwave reception system. Among other problems, parabolic antennas are relatively expensive, and are not sufficiently stable in low winds to guarantee consistent signal reception and hence picture quality. Thus, they are not particularly suitable for everyday use in home or other commercial TV reception systems.
Stripline or microstrip antennas for microwave transmission or reception are known in the art. Such antennas are shown, for example, in UK Pat. No. 1,529,361 to James and Wilson, U.S. Pat. Nos. 3,995,277 to M. Olyphant, Jr., 3,987,455 to M. Olyphant, Jr. and 3,803,623 to L. Scharlot, Jr. In all of these prior patents the antenna structure consists of a laminate structure of a dielectric material with an electrically conductive ground plane on one surface of the dielectric and a stripline or microstrip pattern on the other surface of the dielectric. It is well known that the properties of the dielectric material are important to the performance of the antenna, especially the properties of dielectric constant and dissipation factor. Those considerations make these conventional microstrip antennas practicably unsuitable for TV receiver only (TVRO) antennas because they severely limit the choice of suitable dielectric materials to very expensive materials, especially when one considers that a TVRO antenna must be relatively large, such as on the order of a square structure 30 to 40 inches on each side or a circular structure having a diameter of 30 to 40 inches. Also, since TVRO antennas will be used outdoors, they must be weatherized to protect them from exposure to the elements. This is particularly so with the conventional prior art stripline or microstrip antennas where the circuit pattern and the ground plane are on the exterior of the dielectric surfaces. This weatherizing requirement further adds to the economic and practical problems of using prior art microstrip antennas in TVRO systems.
The combined requirements of electrical properties and weathering resistance limit the choice of dielectric materials that may be effectively employed in a practicable TVRO antenna if one were constructed in accordance with conventional prior art techniques. The combined requirements of electrical properties and weathering resistance limits the choice of dielectric materials. Low loss ceramics would offer good performance for the dielectric material, but the cost and limited size of ceramic substrates would rule them out. PTFE (polytetrafluoroethylene) based substrates or substrates based on other fluoropolymers would also be acceptable choices from the standpoint of dielectric properties, but the cost of such substrates would make them unsuitable for home and general commercial use. Thus, because of the economic and other practical drawbacks, the art has not developed a commercially practicable and acceptable planar TVRO antenna.
The microstrip antennas disclosed in the previsously mentioned UK Pat. No. 1,529,361 and U.S. Pat. Nos. 3,995,277, 3,987,455 and 3,803,623 may be described, in general terms, as having a dielectric body with a ground plane on one surface and a radiator pattern on the other surface. It is known that antennas of this type can experience a problem of surface waves which are generated at the boundaries of the dielectric support for the radiator and air. These surface waves will travel between radiators and constitute a power loss in the system and impair the quality of beam formation.